The present embodiments relate to a low noise memory array architecture suitable for Dynamic Random Access Memories (DRAM), Static Random Access Memories (SRAM), and other memory configurations having complementary bit lines.
Generally, array noise includes any signal transitions that would degrade a data signal either before or during amplification. These signal transitions may be on adjacent bit lines, word lines, column select lines, or other relatively nearby signal lines. The signal transitions are often coupled to the data signal through parasitic capacitance due to the close proximity of the interfering signal lines. Array noise problems, therefore, tend to increase with decreasing feature sizes, since the desired data signal decreases while the parasitic capacitance increases.
FIG. 1 is a diagram of a memory array of the prior art showing adjacent bit lines having a folded bit line architecture in a triple twist configuration. The memory array includes sense amplifiers 100, 110, and 120. In the following discussion a signal bit line includes at least part of an active memory cell that is being read or written. A reference bit line has a capacitance that is substantially equal to the signal bit line capacitance and is set to a reference voltage. Signal bit lines of each sense amplifier are depicted as bold lines, and reference bit lines are depicted as normal weight lines. For example, sense amplifier 100 is connected to reference bit line 102 and signal bit line 104. Sense amplifier 110 is connected to reference bit line 112 and signal bit line 114. Sense amplifier 120 is connected to reference bit line 122 and signal bit line 124.
Referring to FIG. 2, there is a cross-sectional view of adjacent bit line conductors as in FIG. 1 showing parasitic capacitors. Here and in the following discussion, parasitic capacitors are not separate discrete components but have non negligible capacitance due to the proximity of adjacent bit line conductors. The diagram shows adjacent bit line conductors 204, 206, and 208. Parasitic fringe capacitor CF 214 is between adjacent bit lines 204 and 206 Likewise, parasitic fringe capacitor CF 216 is between adjacent bit lines 206 and 208. There are also upper 200 and lower 202 conductors adjacent the bit lines. Conductor 200 may comprise overlying word lines of other signal lines. Conductor 202 may be a substrate or other signal lines. Parasitic planar capacitor 210 is between conductor 200 and bit line 206 and has a value of αCP, where α is a real number between 0 and 1. Parasitic planar capacitor 212 is between conductor 202 and bit line 206 and has a value (1−α)CP. For the purpose of the following discussion the total parasitic fringe capacitance to each bit line is 2CF, and the total parasitic planar capacitance to each bit line is CP as will be discussed in detail.
Referring now to FIG. 3, there is a diagram of a memory array as in FIG. 1 showing major parasitic fringe capacitors C0 through C7 for a triple twist bit line configuration. Word line 300 selects three memory cells represented by small circles. These memory cells each develop data signals on their respective signal bit lines shown in bold and connected to respective sense amplifiers 100, 110, and 120. Reference bit lines shown in normal line weights are also connected to respective sense amplifiers 100, 110, and 120. For the case where all three memory cells store a 1, there is no charge transfer through parasitic capacitors C0 or C6, because the change of voltage with respect to time (dv/dt) on all signal bit lines is substantially the same Likewise, there is no charge transfer through parasitic capacitors C2 or C4, because dv/dt on all reference bit lines is substantially the same. However, the signal bit line 302 of sense amplifier 110 couples charge to the reference bit lines of sense amplifiers 100 and 120 through parasitic capacitors C3 and C5, respectively. In a similar manner, the signal bit lines of sense amplifiers 100 and 120 couple charge to the reference bit line 304 of sense amplifier 110 through parasitic capacitors C1 and C7, respectively. The total coupling to the reference bit line 304 of sense amplifier 110, therefore, is the coupling from the signal bit line 302 (CF) plus the coupling through parasitic capacitors C1 and C7 (CF/2), where CF is the fringe capacitance between two adjacent bit lines for their total length.
FIG. 4 is a schematic diagram showing the capacitive coupling to the reference bit line 304 of sense amplifier 110 (FIG. 3). When word line 300 activates the memory cells of FIG. 3, a voltage V(1) is developed on each signal bit line across planar parasitic capacitor CP 400. A fraction of this voltage is coupled through parasitic fringe capacitor 402 (1.5 CF). The noise voltage on reference bit line 304, therefore, is equal to V(1)/(1+2CP/3CF). This noise voltage (Vn) on reference bit line 304 reduces the difference voltage between signal bit line 302 and reference bit line 304. For example, when CP is equal to CF, the resulting signal to noise ratio (V(1)/Vn) is 1.67. Under worst case conditions, this may require slower sensing by amplifier 110 and may result in read errors. Thus, there is a need for noise reduction in memory arrays.
Referring to FIG. 5, there is a diagram of a memory array having an open architecture in a cross point configuration. Here and in the following discussion a cross point configuration means that a memory cell is placed at every intersection of a word line and bit line. Typically less area is required for a cross point array than for a folded array. According to previous estimates, a memory cell in a folded array may require 8F2 as compared to a memory cell in a cross point array that requires 4F2 to 6F2 of cell area, where F is a minimum feature size. FIG. 5 illustrates major parasitic fringe capacitors CF for an open architecture in a cross point configuration. Word line 512 selects four memory cells represented by small circles. These memory cells each develop data signals on their respective signal bit lines shown in bold and connected to respective sense amplifiers 500 and 502. Reference bit lines shown in normal line weights are also connected to respective sense amplifiers 500 and 502. For the case where each of memory cells 504 store a 1 and memory cell 510 stores a 0, there is substantial charge transfer through parasitic capacitors CF, due to the change of voltage with respect to time (dv/dt) between signal bit lines. FIG. 6 is a schematic diagram showing the capacitive coupling to the signal bit line of sense amplifier 502 (FIG. 5). When word line 512 activates the memory cells of FIG. 5, a voltage V(1) is developed on signal bit lines of sense amplifiers 500 across planar parasitic capacitor CP 600. A fraction of this voltage is coupled through parasitic fringe capacitor 602 (2 CF). The noise voltage imparted to the signal bit line 508 of sense amplifier 502, therefore, is equal to V(1)/(1+CP/2CF). This noise voltage (Vn) on the signal bit line reduces the difference voltage between the signal bit line and the reference bit line. For example, when CP is equal to CF, the resulting signal to noise ratio (V(1)/Vn) is 1.5. This is worse than the folded architecture of FIG. 3 and may require even slower sensing by amplifier 502. Thus, there is an even greater need for noise reduction in memory arrays having a cross point configuration.